1. Field of the Invention
This invention relates to facsimile systems. More particularly, this invention relates to imaging arrays having a plurality of energy sources and means for accessing and activating the energy sources.
2. Description of the Prior Art
In a conventional facsimile transceiver, transmission of data is performed by scanning a datacontaining document line by line and by converting the light reflected from the scanned portions of the document into a series of corresponding electrical signals. Those signals are transmitted, typically over a conventional telephone line, to a remote facsimile transceiver where the signals are processed to reproduce the information on a suitable print medium.
In the transmitter portion of a conventional transceiver, a light beam, generally produced by a cathode ray tube, is caused to scan along one axis across the document while the document is incrementally moved along on axis perpendicular to the direction of scanning. In that manner, the entire document is effectively traversed with parallel scans.
In the receiver portion of a transceiver, several electronic and mechanical techniques are in common use for the purpose of processing received data to produce an image on a print medium. One such technique employs a mechanical stylus operating in response to the received data signals to print the desired pattern on specially prepared paper. Another technique employs a print paper which contains overlaying black and white layers. Portions of the white layer are selectively burned or etched away by the use of an electrically charged stylus that operates in accordance with the received data signals, thereby developing the desired pattern.
Still other known facsimile receivers employ a light source, such as a cathode ray tube. The light beam is modulated in accordance with the received data signals and is scanned over a suitably treated medium to form a pattern of locations on the paper. The medium may be photograhic film which is subsequently processed to produce a permanent image. U.S. Pat. No. 3,924,061 issued to Tregay et al on Dec. 2, 1975, and U.S. Pat. No. 3,869,569 issued to Mason et al on Mar. 4, 1975, are examples of such facsimile receivers.
In yet other receivers, the light source employed is a laser beam. Such receivers operate in a manner similar to those employing a cathode ray tube but the beam is generally applied to a print medium other than photographic film. In "An Experimental Page Facsimile System," by H. A. Watson, Bell Laboratories Record, March 1975, page 153, a laser receiver is described where the print medium is bismuth film.
A common thread to the above-described techniques is the use of a single light source to form a two-dimensional received image and the use of analog modulation of the light source intensity to control the gray levels of the resultant pattern.
In a slightly different field of art, a two-dimensional display is achieved by the use of a plurality of light sources arranged in a matrix configuration. Of common knowledge are gaseous displays and light-emitting diode (LED) arrays which are used extensively to display alphanumerics. For example, LED arrays having groups of diodes preselectively interconnected and activated as a group (e.g., seven segments) are commonly used in calculator displays. A slightly different LED array arrangement is disclosed in U.S. Pat. No. 3,800,177 issued to Russ on May 26, 1974, where the LED array is arranged in a horizontal and vertical address matrix and where the activation of a particular horizontal and vertical address line pair activates a single LED. Whatever the application, LED arrays have heretofore been used only to display a twotone image (generally red on a black background).
Different arrangements, such as described in U.S. Pat. No. 3,863,023 issued to Schmersal et al on Jan. 28, 1975, provide multitone displays. In the arrangement described by Schmersal, multitone operation (graduated intensity levels of a particular tone such as green, red, black, etc.) is achieved in a gaseous discharge panel having a multiple number of memory planes. In particular, a number of gray level ranges are defined and an equal number of memory planes are employed, with each memory plane having the same number of storage areas as the number of storage and discharge areas in the display panel. (For simplicity, the term "gray level" is employed in this disclosure regardless of the actual hue employed). In generating the data base, a pictorial field is scanned with a vidicon tube and the elements of the signal corresponding to the picture elements are digitized according to the gray level range in which they fall. In forming a replica of the image field, the signals representing the digitized picture elements are fed in sequence to the various memory planes corresponding to each gray level. The brightness level of each plane is controlled by the storage characteristics of each memory plane and the duration of excitation.
The Schmersal et al apparatus is cumbersome because it requires the use of hardware that is both bulky and expensive. Additionally, the memory planes of Schmersal must be accurately aligned and separately driven with high voltages.
Another method for effecting different gray levels is described in U.S. Pat. No. 3,604,846 issued to Behane et al on Sept. 14, 1971. In accordance with the teachings of Behane et al, gray level graduations can be achieved by subdividing the area of each picture element (pel) into a plurality of subareas, e.g., a 3 .times. 3 matrix having nine subareas, and by marking black a number of the subareas in accordance with the gray level desired. Thus, white pels are obtained by marking black none of the subareas, progressively darker gray pels are obtained by marking black greater numbers of subareas, and black pels are obtained by marking black all nine subareas.
The Behane method is useful in situations where high receiver resolution is inherent in the system and is, therefore, obtained at low cost. Where high resolution is not inexpensively available, this method becomes too costly because for each macroscopic resolution element (pel), a large plurality of microscopic resolution elements (the subareas) must be employed.
To circumvent the difficulties and disadvantages of pg,6 prior art facsimile systems, a new facsimile receiver has been invented and disclosed in a patent application filed concurrently with this disclosure (Goldschmidt et al Ser. No. 750,273, filed on Dec. 13, 1976. The new facsimile receiver employs a plurality of energy sources arranged in an array. The energy sources employed are two-level (ON/OFF) sources, such as light-emitting diodes. The sources are connected to a multiport memory which activates the plurality of energy sources simultaneously.
In some applications, the energy sources are formed in a line array which represents a single scan of the transmitted facsimile data. To generate a two-dimensional pictorial field, the print medium is passed along an axis perpendicular to the line array as successive scans are illuminated. In such applications, it is extremely important that all of the energy sources should provide exactly the same energy outputs in response to the same stimuli.
It is an object of this invention, therefore, to provide means for effectively controlling and equalizing the output of the energy sources in the array.
It is another object of this invention to provide means for equalizing the output of the energy sources in real time.